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Potassium dichromate, K2Cr2O7, forms large, red crystals, which are soluble. Sodium dichromate is often used in place of the potassium salt, because it is more soluble. Both chromates and dichromates are used in dyeing, in calico-printing, and as oxidizing agents. Potassium dichromate is used in photography and to prevent polarization in the "dichromate" battery.

When a chromate solution is treated with an acid, a dichromate is formed. The color is changed to red, owing to the formation of Cr2O7 ions. Conversely, a base changes a dichromate to a chromate. We can understand the relation of chromates to dichromates better if we write the formulas of the corresponding acids as hydrates of their anhydrides, just as we did with the manganese acids (cf. § 507).

(1) K2CrO4+2 HNO32 KNO3+H2CrO, (i. e., CrO3.HO). (2) K2CrO1+CrO3.H2O→→→ K2Cr2O;+H2O.

Potassium dichromate is thus an acid salt, but the form KHCrO4 loses water.

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While the reverse action, producing KHCrO4, is small, it is appreciable; so that when potassium dichromate solution is added to the solution of a ionic metal that forms an insoluble chromate, the chromate of the metal is precipitated.

(1) K2Cr2O7+H2O (2 KHCrO1).

(2) 4 AgNO3+ (2 KHCrO1) →→→→ 2 Ag:CrO4 | +2 KNO3+

2 HNO3.

When a base is added to a dichromate, the reversible equation,

K Cr2O+H2O2 KHCrO4,

OXIDATION BY CHROMATES.

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is made complete, by the replacement of the H of KHCrO4 by a metal. Hence a chromate is formed.

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509. Oxidation by Chromates and Dichromates. When chromic compounds are oxidized to chromates, a base is added, as well as an oxidizing agent; hence when chromates or dichromates are used as oxidizing agents, i. e., are reduced, the operation is carried out in acid solution. The fundamental reaction is the change of chromium trioxide to chromic oxide, with loss of oxygen (cf. § 507).

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The oxygen unites with the reducing agent, while the chromic oxide reacts with the acid, giving a chromic salt. The reaction between potassium dichromate, dilute sulphuric acid, and sulphurous acid (sulphur dioxide) is as follows:

(1) K2Cr2O7+H2SO1

(2) (H2Cr2O7)

(H2Cr2O7)+K2SO1.

H2O+(2 CrO3).

(3) (2 CrO3) →→ (Cr2O3)+(3 0).

(4) (CrO3)+3 H2SO, Cr2(SO4)3+3 H2O.

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(5) 3 H2SO3+(3 0) - →3 H2SO4.

When the solution is evaporated at the ordinary temperature, chrome-alum is formed (cf. § 506).

When alcohol is the reducing agent, equation (5) is:

3 C2H2O+(30) →→→3 C2HO ↑ (aldehyde) +3 H2O.

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Hydrochloric acid and potassium dichromate give chlorine (cf. § 116). In this case, HCl takes the place of H2SO4 in equations (1) and (4), and of H2SO3 in equation (5).

Hydrogen peroxide oxidizes a 'dichromate to perchromic acid, which is extracted from water solution by ether, and colors the ether blue (cf. § 340).

510. Exercises.

1. Give the equations for: —

(a) Heating MnO and MnO2 in air.

(b) Oxidation of Mn(OH)2 by the air.

(c) Action of MnO2 with concentrated H2SO4. With HCI. 2. Give partial and complete equations for:

(a) Heating a mixture of Mn(NO3)2, KOH, and KCIO3. (b) Treating the following with KMnO, in dilute sulphuric acid solution: formic acid, sulphurous acid, ferrous chloride. (c) Action of HCl upon solid KMnO4.

(d) Preparation of Cr2O3 from K2Cr2O7 and NH4Cl (§ 505). Remember that NH4Cl → NH3+HCl.

(e) Reaction between solutions of Cr(NO3)3 and K2CO3. (f) The same for solutions of Cr2(SO4)3 and (NH4)2S.

(g) Action of HCl upon solid K2Cr2O7.

(h) Reaction when H2S is passed through a dilute sulphuric acid solution of KMnO4. Through one of K2Cr2O7.

(i) Preparation of K2CrO4 from CrCl. From Cr2(SO4)3.

3. Exactly 20 c.c. of a solution containing 15.8 g. of KMnO4 per liter were needed to react completely with some H2O2 solution. How much H2O2 was there?

4. If 0.056 g. of iron is dissolved in dilute sulphuric acid, how much KMnO4 is required to oxidize the resulting FeSO4 to the ferric state?

5. How could you prepare lead, calcium, and silver chromates? Give the equations.

6. What is meant by the "polarization" of a voltaic cell? To what is it due? How does K2Cr2O7 prevent it? What chromium compound is formed in the " dichromate "battery?

CHAPTER XXXVII.

LEAD, TIN, AND PLATINUM.

511. Occurrence and Preparation of Lead. Lead occurs chiefly as galena, PbS, and is obtained from it by the following processes:

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(1) If the ore is of high grade, it is first roasted in a reverberatory furnace (cf. § 462). By this operation part of the ore is changed to the oxide, PbO, and part to the sulphate, PbSO4, while some remains unchanged.

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After the oxidation has gone far enough, the furnace doors are closed, and the mixture is heated without the admission of more air. The lead oxide and sulphate then react with the unchanged sulphide as follows:

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(2) If the ores are poor, they are reduced in a blast-furnace, or heated with iron.

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(3) If there is enough silver to pay for its extraction (base bullion; cf. § 462), the Parkes process is used.

(4) Lead may also be made by the electrolysis of a mixture of galena and dilute sulphuric acid. About 970,000 tons of lead were produced in 1910; 372,000 tons in the United States.

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512. Properties and Uses. Lead is a soft, bluegray metal, having a high luster which is soon tarnished by oxidation. The corrosion does not penetrate, as with iron. Lead is not very malleable, or ductile, but it can be rolled into sheets, and forced, while hot, through steel dies, forming lead pipe. It melts at 327° C.

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Lead (i. e., lead ions, Pb) is readily displaced from the solutions of its salts by such metals as zinc and iron (cf. § 180). It often separates in a branching, crystalline form, known as a lead tree. Nitric acid reacts readily with lead, forming the nitrate. Dilute sulphuric attacks it very slowly; the hot, concentrated acid acts as upon copper (cf. § 265). When lead is boiled with concentrated hydrochloric acid, hydrogen is formed, but very slowly. Many organic acids, e. g., acetic acid, act upon it, in the presence of air, to form soluble salts.

All compounds of lead are poisonous. When even small amounts of lead are taken regularly into the body, as in drinking water, the lead accumulates until it causes sickness. Painters are subject to "painter's colic," because of the lead compounds in paint. Lead is almost unaffected by pure water, if air is not present; but air and soft water together produce lead hydroxide, which is slightly soluble. The presence of carbon dioxide makes lead react even more readily with water. Hard water acts upon lead very slightly, because a coating of lead carbonate and sulphate soon protects it from further action. Great care should be exercised in using water that has run through new lead pipes.

Besides being used for conveying water, lead pipes are used as sheaths for the cables of telephone wires. In sheet form, the metal is used to line the "leaden chambers" (cf. § 262), and the inside of vats and tanks used for other chemical processes. Large quantities of it are made into shot and bullets, the plates

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